1. Field of the Invention
The present invention is related to a control circuit for a heat-dissipation fan dissipating heat from an electronic device and more particularly to a control circuit for a heat-dissipation fan which can efficiently improve power savings and extend the usable life of the fan.
2. Description of the Prior Art
A typical control circuit for controlling the rotational speed of a heat-dissipation fan operates by temperature control. Accordingly, the fan is rotated at full speed to provide higher heat dissipation effect when the ambient temperature of the device reaches a predetermined higher threshold (extremely operating circumstance) and is reduced to a lower speed (usually about 40% of the full speed) when the ambient temperature falls below a pre-determined lower threshold (normal operation). Alternatively, the speed of the fan can be changed linearly between the low speed and the high speed in linear relationship with the change in the ambient temperature. Nevertheless, such a control circuit for the fan still has some drawbacks. The fan requires significant power at all times, since it is running at low speed during times when the ambient temperature is normal or even low. Additionally, this constant use may cause an overload problem or shorten the usable life of the fan. Preferably, the fan should be stopped at low ambient temperatures, thus not wasting energy on cooling when none is needed.
The primary object of the present invention is to provide a control circuit for a heat-dissipation fan that can efficiently improve power savings and extend the usable life of the fan. The present invention comprises a control circuit for controlling the rotational speed of a heat-dissipation fan for an electronic device, said fan having a permanent magnet and a coil means, said control circuit comprising: a temperature sensing circuit for sensing the ambient temperature of the device and providing a temperature signal related thereto; a comparator circuit for receiving said temperature signal and for comparing said temperature signal with a predetermined threshold and providing a comparison signal related thereto; a circuit for detecting the phase of the fan magnet and for providing a phase signal related thereto; and a drive circuit for receiving said phase signal and said comparison signal and for driving said coil means to control the rotational speed of said fan, when the ambient temperature is above said threshold, to change substantially linearly with respect to the difference between the ambient temperature and the threshold and, when the ambient temperature is below said threshold, to stop the rotation of the fan. Advantageously, the predetermined threshold is chosen to correspond to a fan speed appropriate to dissipate the heat from the device in its normal operating state.
In a preferred embodiment, when the ambient temperature is below the threshold, the fan is driven by the drive means with sufficient energy to put the fan into a non-rotational standby mode, thus improving power savings and extending the usable life of the fan.
Other objectives, advantages and novel features of the invention will become more apparent from the following detailed description and the accompanying drawings.